1. Field of the Invention
The invention disclosed herein relates to a method for calibrating downhole tools and, in particular, to downhole tools configured to measure electrical resistivity by induction.
2. Description of the Related Art
A borehole is typically drilled into a geologic formation for various applications such as carbon sequestration, geothermal production, and hydrocarbon exploration and production. In order to efficiently use expensive drilling resources, it is important to acquire detailed information concerning the properties of the geologic formation.
A logging or downhole tool can be used to acquire this information. The tool is disposed in the borehole by wireline for wireline logging or in a collar attached to a drill string for logging-while-drilling applications. Many different types of tools can be used to measure different properties of the formation.
One type of tool is an induction logging tool used to measure the resistivity of the formation as a function of depth in the borehole. Variations in the measured resistivity can be plotted or displayed to produce a resistivity image of the formation. In order to measure the resistivity, the induction logging tool transmits electromagnetic waves into the formation using a transmitter coil. The electromagnetic waves induce circulating currents in the formation. The magnitude of the circulating currents vary in response to variations of the resistivity of the formation. The circulating currents in turn emit electromagnetic waves that induce a signal in a receiver coil at the tool. The received signal is related to the magnitude of the circulating currents and, thus, the resistivity of the formation.
Because the induction logging tool must be sensitive to low levels of electromagnetic waves induced by the circulating currents in the formation, it is important for the tool to be calibrated properly. A proper calibration will exclude any influences from a calibration environment that may interfere with the calibration due to the sensitivity of the tool. It would be well received in art if the calibration of induction logging tools could be improved.